Cat’s Pee Or Passionfruit? Making Sauvignon Blanc Taste and Smell Better
Sauvignon blanc grapes growing at one of Brancott Estate Wines’ Marlborough vineyards. Photo: Brancott Estate Wines.
When Victoria University chemist Dr Robert Keyzers opens his lab fridge, the room is filled with the smell of cat’s urine. It’s not a pleasant smell under any circumstances – but especially not when it’s associated with wine.
Sauvignon blanc – bottles of which fill Dr Keyzers’s fridge – is New Zealand’s most successful wine, responsible for more than $1 billion of sales.
The volatile, sulphur-containing molecules that give Sauvignon blanc its distinctive smell are called thiols. At low levels, thiols give the wine its characteristic gooseberry, tropical and passionfruit aroma. But at high levels they can make the wine smell like cat’s urine. While thiols are crucial to the aroma and flavour of the wine, no one knows quite how they are created – free thiols are present in wine but not in grapes. Understanding how thiols are synthesised, and at what stage of the winemaking process, is now a major area of wine research in New Zealand.
One of the focuses of Dr Keyzers’ vine-to-wine research project is thiol analysis, which is currently done with a technique that uses toxic organomercury compounds.
“We’re trying to develop better methods to do the thiol analysis,” he says.
“Once we’ve got robust numbers on how much thiol is in the wine, we can work backwards to try and track back to what the precursor is.”
Dr Keyzers and his student, Danica Carter, are developing a thiol-sensitive fibre that can be used for solid-phase micro-extraction, a safe and common laboratory technique used widely around the world. To test the fibre, Dr Keyzers is collaborating with Brancott Estate Wines (formerly Montana Wines), which has provided him with access to the sophisticated robotics technology at one of their Marlborough laboratories.
Dr Rob Keyzers’ investigations of the links between grape metabolites and wine flavour and aroma include the study of methoxypyrazines, molecules that can taint the flavour of Cabernet Sauvignon wines.
Thiol analysis is just one stage of a larger project to develop designer wines.
“We want to be able to design specific flavour and aroma profiles to target specific export markets,” says Dr Keyzers.
That means getting the flavour of the grape right.
“If we could analyse a grape really early on in the growth cycle, then tell the vineyard manager to ‘give that vine more water’, or ‘give that vine more phosphate’, for example, they could produce a grape with specific chemical composition that could produce a wine with a flavour and aroma profile to hit specific target markets.”
Thiols are widespread in foods as well as wines, and the technology Dr Keyzers is developing could eventually be used to analyse taints and contaminants in a range of white and red wines as well as foods like cheeses and meat products.